Image formation in second-harmonic near-field microscopy

A macroscopic self-consistent approach that enables one to rigorously descr ibe image formation in scanning near-field optical second-harmonic generati on microscopy is developed. The self-consistent second-harmonic field is de termined by taking into account both the linear and nonlinear contributions in the effective current, i.e., the currents generated by the self-consist ent fields at the fundamental and second-harmonic frequencies. The self-con sistent problem for both frequencies is solved exactly by use of the diagra m technique adapted from quantum electrodynamics. Preliminary numerical res ults calculated for a rectangular object are presented and compared with ex perimental observations. It is demonstrated that the optical contrast and t he spatial resolution are significantly better in the second-harmonic image s than in the images obtained at the fundamental frequency.